The incorporation of electronic devices within pneumatic tyres is taking a greater importance in order to increase safety of vehicles. Tyre electronics may include sensors and other components suitable for obtaining information regarding the behavior of a tyre, as well as various physical parameters thereof, such as for example temperature, pressure, number of tyre revolutions, vehicle speed, etc. Such information may become useful in tyre monitoring and/or alarm systems. Furthermore, active control systems of the vehicle may be based on information sent from sensor devices included within the tyres. Typically, wireless transmission is employed in order to send tyre performance information outside the tyre, to a receiver disposed on the vehicle, so that such electronic devices disposed within the tyre typically include a transmitter associated to an antenna. A microprocessor is also typically employed, in order to collect and process the signals coming from the performance sensors, before transmission.
An important parameter to be monitored for a proper use of a tyre is the load to which the tyre is subjected when fitted on a vehicle. This operating parameter of the tyre is very important in order to correctly set the inflation pressure of the tyre, as well as in order to correctly tune active control systems of vehicles. Another important parameter is the length of the contact patch, i.e. of the contact region between the tyre and the road surface.
WO 05/005950 discloses a method for determining a load exerted on a tyre fitted on a vehicle by measuring the amplitude of the deformation in radial direction to which a portion of the tread area of the tyre is subjected when such portion passes in correspondence of the contact region between the tyre and the road (“radial deformation”), and by relating such amplitude to the rotation speed and to the inflation pressure of the tyre. In the embodiments disclosed in WO 05/005950, the radial deformation is detected by means of a radial accelerometer secured to the inner liner of the tyre.
WO 05/042281 discloses a method for determining a load exerted on a tyre fitted on a vehicle during a running of said vehicle on a rolling surface, comprising providing a concave upwards function Fz=Fz(PLc) of said tyre load versus a length (PLc) of a contact region between said tyre and said rolling surface; estimating said length substantially at the equatorial plane; and deriving the tyre load corresponding to said estimated length from said function. In the embodiments disclosed in WO 05/042281, the contact length is estimated by acquiring a tangential acceleration signal from a tangential accelerometer, and by measuring a distance between a maximum value and a minimum value of said tangential acceleration signal.
Integrated tyre electronics systems have conventionally been powered by a variety of techniques and different power generation systems.
A typical solution for powering tyre electronics systems is the use of a non-rechargeable battery, which may cause inconveniences to a tyre user since proper electronics system operation is dependent on periodic battery replacement. As a matter of fact, batteries tend to deplete their energy storage quite rapidly when powering electronic applications characterized by complex levels of functionality. Furthermore, conventional batteries typically contain heavy metals that are not environmentally friendly and which present disposal concerns. Moreover, performances of conventional batteries are often influenced by temperature: in particular, the functioning of such batteries is not reliable at low temperatures.
Another known method for powering tyre monitoring systems is a coupling of radio-frequency (RF) power between an antenna disposed on the vehicle in close proximity with another antenna included within the electronic device disposed in the tyre. This typically requires antennas disposed in vehicle portions frequently exposed to damage from road hazards, and thus may lead to many drawbacks.
The use of piezoelectric elements has also been proposed for powering tyre monitoring systems. Piezoelectricity is a property of certain materials, such as quartz, Rochelle salt, and certain solid-solution ceramic materials such as lead-zirconate-titanate (PZT), of generating electrical charge when mechanically stressed.
WO 2005/067073 discloses a tyre comprising a piezoelectric flexing element associated with an energy storage device (e.g. a capacitor). The piezoelectric flexing element is mounted in cantilever fashion in a housing so as to be positioned substantially along a plane orthogonal to a radial direction of said tyre and, so that a first end of the piezoelectric element is restrained to the housing. A loading mass is coupled to the second end of the piezoelectric flexing element. A small gap is formed between the inner walls of the housing and the outer surface of the loading mass, in order to allow limited flexure of the piezoelectric element. The housing including the piezoelectric element is mounted in a tyre portion in correspondence of a tread area of the tyre, preferably on the inner surface of the tyre. The piezoelectric element flexes under the action of the radial acceleration when the tyre rotates. The loading mass and the gap are chosen to obtain: a) small entity oscillations of the flexure element substantially during a complete revolution of the tyre, when the tyre rotates at low speed; b) large entity oscillations of the flexure element substantially only during the passage of the tyre portion including the piezoelectric element in the contact patch.
WO 2006/072539 discloses a configuration of a spring element as a rod spring, torsion spring or leaf spring, in which the free end of the spring element carries a seismic mass, to which an impulse is applied by the rolling of the tyre. The combination of the seismic mass with the spring element produces a spring-mass oscillator. Oscillation is generated by the motion whereby the tyre module, during the rotation of the tyre, describes a straight line as it passes through the contact area, and a circular path as it moves out of the contact area. In the circular path, centrifugal force acts on the seismic mass whereas, under ideal conditions, no force will act on the seismic mass in the contact area. Centrifugal force will displace the spring-mass oscillator, which will then return to its position of rest as it passes through the contact area. According to WO 2006/072539, in order to produce a complete module for the recording of tyre condition variables, an analysis unit for the evaluation of electrical output signals from the conversion unit, or other sensor data, can be connected to the conversion unit. It is then possible, e.g. on the basis of the time interval between two acceleration pulses, to determine the length of the contact area, i.e. the size of the tyre contact surface. According to WO 2006/072539, it is also possible to determine the speed of rotation of the wheel, or the wheel load.